This disclosure describes a method and an apparatus for improving light exposure, e.g., ultraviolet exposure of photo-curable printing plates, e.g., photopolymer flexographic printing plates, letterpress plates and other polymer printing plates, as well as polymer sleeves and polymer coated printing cylinders. Photo-curable, of course, means curable by photons, e.g., light, e.g., light in the ultraviolet range or some other range.
Photopolymer plates have found a broad range of applications. A variety of different methods can be applied for transferring an image for printing, e.g., in the form of imaging data, to a polymer plate. For example, an image mask, which can be a film applied to the surface of the plate while the plate is exposed, or can be an ablatable layer directly on top of the polymer surface that is laser ablated to form a mask.
By a digital plate is meant a plate that is exposed to imaging data by ablating a mask material that is on the plate, e.g., by exposure to laser radiation in an imaging device. The process of producing a digital plate is called a digital process herein. After exposure, the plate needs light, e.g., UV light for curing. It is known to carry out in-line curing on the same apparatus that is used to expose the plate to imaging data.
Polymer printing plates are three dimensional, that is, include a depth dimension from the printing surface. Small printing details on the plate's surface carry ink for printing. For line art and other small features, it is common to have printing features that are substantially flat. Such a feature is called a flat top herein. It is also common to produce features on digital plates tend to have rounded surfaces that extend down in depth. Such a feature is called a round top herein.
Flat tops allow much higher ink transfer volume in the print compared to round tops, especially for solid areas and line work that have been screened with a high frequency pattern. However, for some applications, flat tops have some disadvantage. Screen dots printed with flat tops for low percentage screen dot areas in highlights of a print may end up with too much ink transfer.
Many printing plate jobs include line work as well as screened areas. Pure flat top structures may not be suitable for use in such jobs without compromising the quality of the screened areas of the job. Too much ink transfer in highlight regions means the lowest printable gray scales are higher than with conventional digital printing with round tops. This limits the gray scale range available.
To overcome this problem, some printing plate fabricators use two sets of plates for the same job: a first set of plates, e.g., one plate for each color for line work and a second set of plates, e.g., one plate for each color for the screen portion of the job.